The typical hard disk drive includes a head disk assembly (HDA) and a printed circuit board (PCB) attached to a disk drive base of the HDA. The head disk assembly includes at least one disk (such as a magnetic disk, magneto-optical disk, or optical disk), a spindle motor for rotating the disk, and a head stack assembly (HSA). The printed circuit board assembly includes electronics and firmware for controlling the rotation of the spindle motor and for controlling the position of the HSA, and for providing a data transfer channel between the disk drive and its host. The HSA typically includes an actuator, at least one head gimbal assembly (HGA), and a flex cable assembly. During operation of the disk drive, the actuator must rotate to position the HGAs adjacent desired information tracks on the disk.
Each HGA includes a head for reading and writing data from and to the disk. In magnetic recording applications, the head typically includes an air bearing slider and a magnetic transducer that comprises a writer and a read element. The magnetic transducer's writer may be of a longitudinal or perpendicular design, and the read element of the magnetic transducer may be inductive or magnetoresistive. In optical and magneto-optical recording applications, the head may include a minor and an objective lens for focusing laser light on to an adjacent disk surface. The head is adhered to a suspension assembly that includes a gimbal, load beam, bend region, and swage plate. The suspension acts to preload the head against the surface of the disk. The preload force is often referred to as the “gram load.” Each HGA is attached to the distal end of one of the actuator arms, typically by an attachment process known as “swaging” that involves forcing a slightly oversized ball through a boss in the swage plate to cause the boss to plastically expand within a hole in a corresponding actuator arm.
Generally, the greatest data capacity for a given disk drive design is obtained when there is at least one HGA corresponding to each disk surface, so that there are two HGAs swaged to each actuator arm between disks, and one HGA swaged on each of the uppermost and lowermost actuator arms. This condition is referred to in the art as a “fully populated” disk drive. For example, if a disk drive has a spindle hub that is designed to accommodate three disks, then its actuator would most likely be designed to have four arms (an uppermost arm, two “middle arms” between disks, and a lowermost arm), and such a disk drive would be considered “fully populated” if it included three disks and six HGAs (two swaged to each of the middle arms and one swaged to each of the uppermost and lowermost arms).
Yet not all disk drive customers require or want to pay for the greatest data capacity that a given disk drive design can provide, and the HGA and disk components are among the most expensive components in the disk drive. Therefore, it is commercially advantageous to the disk drive manufacturer to offer so-called “depopulated” versions of a disk drive design, in which one or more HGAs and/or disks are intentionally absent. For example, with reference to the previously described disk drive design that can accommodate three disks and six HGAs, one of the disks and two of the HGAs might be intentionally left out of the assembly to create a less expensive disk drive with ⅔rds the data capacity of the fully-populated version.
A special clamp or disk spacers of a different thickness might be necessary to compensate for the absence of the omitted disk on the spindle hub. Nevertheless, the total cost of engineering development of a family of depopulated disk drives (all based on a single disk drive design for the highest data capacity target) is far lower than would be the cost of independent engineering development of a different design for each of the lower data capacities that might be achieved by depopulation. Therefore, depopulating disk drives is typically the most efficient and practical way for disk drive manufacturers to competitively meet a broad spectrum of customer requirements related to price and data capacity. Accordingly, depopulation may significantly increase disk drive sales without prohibitively increasing engineering development costs.
However, the absence of one or more HGAs on a subset of actuator arms can change the center of gravity of the HSA relative to where the center of gravity would be if the HSA were fully populated. To alleviate this problem, simple dummy masses have been swaged to the actuator arms as a substitute for each absent HGA in depopulated HSAs. Still, even with the dummy masses the dynamic characteristics of the depopulated HSA may be sufficiently different to affect dynamic control, which in turn may lead to reduced system bandwidth and impaired data track seeking and data track following functionality, possibly even increasing data access times and/or error rates when reading and writing data.
Accordingly, there is a need in the art for improved dummy masses for use in depopulated HSAs.